Bug#648367: [dm-devel] Bug#648367: device-mapper devices such as dm-crypt always have rotational=1; should inherit "rotational" setting from underlying devices

[Josh Triplett <josh@joshtriplett.org>]

On Tue, Nov 15, 2011 at 08:34:18AM +0100, Milan Broz wrote:
> On 11/14/2011 03:26 AM, Josh Triplett wrote:
> > OK, I suppose I don't actually care what rotational shows for a
> > device-mapper device backed by rotating media.  The case I care about:
> > when the underlying media has rotational=0, the dm device definitely
> > shouldn't have rotational=1.
> 
> Rotational flag is inherited in DM core, nothing dm-crypt specific.
> And it works (for several kernel releases already):
> 
> 
> create test scsi disk (also with discards)
> # modprobe scsi_debug dev_size_mb=16 sector_size=512 num_tgts=1 lbpu=1
> # grep scsi_debug /sys/block/*/device/model               
> /sys/block/sdj/device/model:scsi_debug 
> 
> Map some crypt device over it
> # echo "password" | cryptsetup create sdj_crypt /dev/sdj
> 
> And now see inherited ROTA flag (it is /sys rotational)
> 
> # lsblk -t /dev/sdj
> NAME               ALIGNMENT MIN-IO OPT-IO PHY-SEC LOG-SEC ROTA SCHED RQ-SIZE
> sdj                        0    512  32768     512     512    0 cfq       128
> `-sdj_crypt (dm-0)         0    512  32768     512     512    0           128
> 
> 
> Please paste lsblk -t output tree if you think there is a bug, do not blindly
> check all dm-X devices queues.

I didn't know about that command; very nice, thanks!  lsblk confirms
that on my system, the physical disk has rotational=0 but the dm-crypt
and LVM devices have rotational=1:

NAME                   ALIGNMENT MIN-IO OPT-IO PHY-SEC LOG-SEC ROTA SCHED
sda                            0    512      0     512     512    0 cfq
├─sda1                         0    512      0     512     512    0 cfq
└─sda2                         0    512      0     512     512    0 cfq
  └─sda2_crypt (dm-0)          0    512      0     512     512    1 
    ├─leaf-swap (dm-1)         0    512      0     512     512    1 
    └─leaf-root (dm-2)         0    512      0     512     512    1 

Does that help?

- Josh Triplett